Patent GB-955092-A (GB)
Title: Improvements in or relating to time division multiplex communication systems
Inventor: FRENCH JAMES ALFRED THOMAS
IPC: H04Q11/04
Filed: 1959-07-24
Abstract: 955,092. Automatic exchange systems. POSTMASTER GENERAL. July 19, 1960, [July 24, 1959], No. 25558/59. Heading H4K. In a pulse multiplex exchange system, incoming lines are connected to GO and RETURN highways 1H, OH via two-way gates such as VT1 opened by pulses PCA and a two-way highway CH, a gate opened by a pulse PG permitting connection between the highway CH and the highway 1H during the early part of a pulse PCA, and a pulse lengthening circuit L2, C5 in the highway OH operating to retime the signal on that highway so that it is applied to CH during the latter portion of the pulse PCA. Completion of a calling subscriber's loop charges condenser C1 (which terminates a lowpass filter L1, C1) negatively, this potential being modulated by the subscriber's speech. The symmetrical transistor gate VT1 is opened by pulse PCA during the early part of which pulse PG opens gate MR1 to allow VT3 to pass a negative pulse to the GO highway 1H. The pulse on the RETURN highway OH causes circuit L2, C5 to ring for half its period, i.e. until the current in L2 attempts to reverse, at which point it is cut off by rectifier MR5. Condenser C5 thereby becomes charged to twice the voltage applied from OH. The condenser C5 is discharged by a pulse PD occurring at the end of the PCA pulse. The potential on C5 is applied via VT5 to a push-pull amplifier VT6, VT7, whereby current flows into or out of Cl until its potential is the same as that of C5. This amplifier is enabled by a pulse PR applied during the latter portion of pulse PCA. The pulse PR persists for a short duration after the termination of PCA, thereby enabling the transistor VT7 to discharge the common highway CH. In certain circumstances, when there is inherent delay in the transfer of pulses from the GO to the RETURN highways gating by the pulse PR may be dispensed with. Where the calling and called parties are in different groups having different highways signals from the respective group GO highways are transferred to the respective group RETURN highways via gates such as PCG5. In the event of calls between parties of the same group three free consecutive pulse positions in the group are seized, the first and third being allocated to one party, e.g. A, and the second to the other party, e.g. B. During the first position VT1 opens and the speech modulated pulse is applied to gate MR14 which is opened by a pulse PIL coinciding with an early part of the channel pulse. The speech modulated pulse is delayed by one channel period at DL1 and then applied to the RETURN highway whence it is passed in the latter half of the second channel pulse to subscriber B via VT2. The pulse PIL is also passed to delay line DL2 whence it is reflected back one channel time later to open MR14 again in time to pass the B subscriber's pulse to the delay line DL1 whose output in the third channel time is applied to subscriber A. The arrangements are such that when no call is being made, or when a call is completely set up the currents through VT1, VT6, VT7 have no direct current component, so that in the absence of speech there is no channel pulse repetition frequency current in the line circuits. Specifications 777,933, 781,561 and 826,631 are referred to.